1. Field of the Invention
The invention generally relates to surface-emitting laser elements, surface-emitting laser arrays, optical scanner devices and image forming apparatuses, and more particularly to a surface-emitting laser element capable of emitting a laser beam in a direction perpendicular to a substrate, a surface-emitting laser array including therein a plurality of such surface-emitting laser elements, an optical scanner device having such a surface-emitting laser array, and an image forming apparatus having such an optical scanner device.
2. Description of the Related Art
In the art of vertical cavity surface emitting lasers (i.e., VCSELs), controlling a high-order oscillation transverse mode of a laser beam and regulating a polarization direction of a laser beam in a desirable direction are important factors in applied VCSEL technologies, and various attempts have been made regarding the control of the high-order oscillation transverse mode of a laser beam and the regulation of the polarization direction of a laser beam in the desirable direction.
Japanese Patent No. 3566902 (hereinafter referred to as “Patent Document 1”) discloses a surface emission semiconductor laser element in which the oscillation transverse mode of a laser beam is controlled. In the surface emission semiconductor laser element, a layer structure of semiconductor materials, where an emission layer is sandwiched between upper and lower reflector layer structures, is formed on a substrate. An opening located above the upper reflector layer structure is coated with upper electrode layers having a circular plan view, and being transparent for the oscillation wavelength of laser light.
Japanese Patent No. 3955925 (hereinafter referred to as “Patent Document 2”) discloses a vertical cavity surface emitting laser device having an opening portion and a semiconductor discontinuity portion formed within the body of the device at a position distant from an edge of the opening portion. The semiconductor discontinuity portion is formed of a slit that is filled with a material differing from the semiconductor material, and a side wall of the semiconductor discontinuity portion has an opening of the slit formed such that the side wall is extended in a desirable direction of polarization of laser light emitted from the laser device that is substantially aligned with a boundary of the discontinuity portion.
Further, Japanese Patent Application Publication No. 2007-201398 (hereinafter referred to as “Patent Document 3”) discloses a surface-emitting semiconductor laser element that includes a substrate, a first multilayer reflection film on the substrate, an active layer having a light emitting central region formed on the first multilayer reflection film, a second multilayer reflection film formed on the active layer having the light emitting central region, and a laser transverse mode adjusting layer formed on the second multilayer reflection film. At least one of the first multilayer reflection film and the second multilayer reflection film includes a quadrilateral current injection region having its intersection of diagonal lines disposed at the light emitting central region of the active layer, and the second multilayer reflection film includes a light emitting window provided in a region corresponding to one of the diagonal lines of the current injection region, and a pair of grooves one groove located at each side of the light emitting window. The laser transverse mode adjusting layer is provided corresponding to the light emitting window and having a peripheral region excluding the central region of the light emitting window having a reflection factor lower than that of a region of the light emitting window corresponding to the light emitting central region of the active layer.
Japanese Patent Application Publication No. 2004-289033 (hereinafter referred to as “Patent Document 4”) discloses a surface-emitting semiconductor laser element that includes a first multilayer reflection film, an active layer formed on the first multilayer reflection film, and a second multilayer reflection film formed on the active layer. At least one layer of the first multilayer reflection film and the second multilayer reflection film includes a first region disposed at least at the part of a region corresponding to a part of the active layer, and having a thickness of substantially λ/4n (λ: oscillation wavelength, n: refractive index); and a second region disposed on a region excluding the first region and having a thickness of substantially excepting λ/4n.
However, although the vertical cavity surface emitting laser device disclosed in Patent Document 2 is capable of regulating the polarization direction, it may still be difficult to suppress the oscillation of the high-order transverse mode of a laser beam due to a change in a laser light confinement effect in a transverse direction based on depths of grooves.
Further, in the surface-emitting semiconductor laser element disclosed in Patent Document 3, if the gap between the grooves is made narrower than the current confined region for regulating the polarization direction, a current passage region may be substantially reduced (narrowed). This has increased electric resistance or electric current density, thereby reducing the service life-span of the laser element.
Moreover, in the surface-emitting semiconductor laser element disclosed in Patent Document 4, the growth of the crystal is temporarily stopped after allowing the crystal to grow up to a layer adjacent to the active layer. The crystals are then allowed to grow again after patterning of the resist and etching of the film are carried out. In this case, when the crystal growth is initiated again, the etched surface of the film may affect the crystal growth, which may bring variability in the characteristics of the laser element or in the control characteristics of the transverse mode of a laser beam. Thus, the laser element may not be suitable for mass production.
The applicants of the present application have conducted various experiments to examine the control of a high-order oscillation transverse mode of a laser beam and the regulation of a polarization direction of a laser beam in a desirable direction, and have found that a polarization mode suppression ratio PMSR may be lowered, in comparison to a case where the reflectance within the emission region is uniform, if a circular low reflectance portion is provided within the emission region of a surface-emitting semiconductor laser element as disclosed in Patent Document 1. Note that the polarization mode suppression ratio PMSR is a ratio of light intensity of a desired polarization direction of laser light to light intensity of a direction perpendicular to the desired polarization direction of laser light.
Thus, even if the surface-emitting laser element is capable of controlling the polarization direction in one direction based on gain anisotropy of the active layer obtained by providing a slanted substrate, the polarization direction may become unstable by providing the circular low reflectance portion in the emission region.
As described above, the applicants of the present application have found that the shape of the low reflectance portion may significantly affect the stability of the polarization.